Electrical generation systems often provide power consumed by power converters, such as three-phase AC/DC or AC/AC power converters. These power converters modify incoming AC (alternating current) power so that it is output with different electrical specifications. For example, electrical generators may supply power with varying operating frequencies, as in the case of generators used aboard certain aircraft, which are driven directly by the operation of the aircraft's jet engine. Because the jet engine's speed also controls the aircraft thrust, the speed must be varied, which results in AC (alternating current) power having a widely-varying primary operating frequency around the base frequency, such as 400 Hz. Unfortunately, the components that are to be powered in many applications are designed to operate within narrow bands of incoming line frequency and voltages. Additionally, designers of power distribution grids impose demanding specifications that must be met by the loads connected to the grid to enable the safe distribution of power thereto. For example, current and voltage distortion [Total Harmonic Distortion (THD)], as well as voltage and current phasing (Power Factor), are often restricted to maximum levels to protect the power generator and the distribution grid, as well as various electronic components and equipment coupled to the grid.
To overcome the problems associated with the varying frequency of the AC power output by such variable frequency power generators, power converters have utilized various power transformer designs. Specifically, power transformers have been designed to provide multiple phase outputs that are rectified and then fed to a DC-link capacitor for supply to a DC bus. However, the rectification of the incoming AC power by the transformer may result in uncorrected and unavoidable power factor shifts, while injecting an undesirable level of harmonic distortion back onto the power grid. To minimize these drawbacks, poly-phase transformers have been developed to increase the frequency of the generated harmonic distortion to a more acceptable level. Because the performance of such transformer designs tends to be dependent on phase voltage balance, line reactors are required to smooth and balance the capacitor charging currents supplied by the poly-phase transformer, which contributes additional weight, size, and cost to the power supply. Thus, while existing transformer designs provide adequate performance, they are deficient with regard to their large size, significant weight, and excessive cost, and do not compensate for load power factor, thus rendering power converters using such transformers undesirable.
In order to improve upon the deficiencies of the transformer-based converters, all-electronic converters, also referred to as active power converters, have been developed. These converters typically operate by transferring the energy available at the incoming main power source to the DC supply or link capacitor by controlling currents through an inductor. Specifically, the active power converter uses a current loop regulator to control the currents through the inductor in accordance with a control loop system that is adjusted based on the converter's operating conditions, to enable its stable operation. However, upon start up of the converter, the lack of voltage potential between the incoming mains and the DC bus, to which the converter is coupled, compromises control loop stability due to the lack of a forcing function (voltage difference) to provide the required gain for the control loop to correctly operate. This can result in initial startup currents that are largely uncontrolled, which places a significant amount of electrical stress on the components of the power converter and the power grid, resulting in a reduction in their operational reliability. Thus, while active power converters can provide power factor correction in a reduced form factor, that is lightweight, they can suffer from startup current spikes, undesirable even-harmonic line distortion, and errors as the operating frequency of the mains power source changes.
Furthermore, active power converters that provide power factor control require line synchronization, which allows the controller to consume current in phase with the line voltage of the power source. Currently, phase-locked loop (PLL) based line synchronization methods are generally used for controlling the power factor in many power applications. During operation, the PLL operates as a timer where synchronization occurs at a phase zero crossing voltage of the input line power, whereby the PLL generates the required steps between successive zero crossings to presume the phase angle of the incoming line power. While the synchronization established by the PLL mitigates any cumulative error, errors during each cycle of the input line power still occur. Thus, while PLLs generally provide acceptable performance under steady state conditions, they can produce significant errors during transient conditions when the frequency of the input line power is changing, which frequently occurs, when a variable frequency power source is used to supply power to the converter.
Therefore, there is a need for a poly-phase AC/DC active power converter that provides a regulated DC voltage power bus, while maintaining a unity or near-unity power factor and providing low total harmonic distortion on both the current and voltage waveforms carried on the lines from a three-phase power source. In addition, there is a need for a poly-phase AC/DC active power converter that utilizes real-time geometric calculations to monitor frequency changes in the power output by a variable frequency power source in order to correct and provide unity or near-unity power factor. Furthermore, there is a need for a poly-phase AC/DC active power converter that can be initially reconfigured at start-up as a boost regulator to increase voltage on a DC-link capacitor in order to provide gain value so as to reduce or eliminate the uncontrolled inrush of current spikes into the power converter.